1. Field of the Invention
The present invention relates to a composition comprising a fire-retardant plastic mixture which contains 55%-75% by weight of boehmite, and to a method of producing a filler material for being worked into the plastic mixture.
2. The Prior Art
DE 33 08 023 CS discloses subjecting hydrargillite to a thermal treatment at 180.degree. C.-200.degree. C. In the process, the hydrargillite changes into boehmite and, in the form of a fire-retardant agent and for pigmentation purposes, it can be worked into plastics in percentages up to 80%.
Furthermore, DE 30 04 310 CS discloses transforming hydrargillite into boehmite at 200.degree. C.-270.degree. C. over a period of 1 to 60 minutes, and then to cool the suspension by reducing the pressure to atmospheric pressure. In the process, the hydrargillite boehmitizes on the surface, so that the main percentage of aluminum hydroxide still contains a bonded percentage of water which is clearly greater than 30%. Furthermore, Table 1 shows that approximately 70% of the boehmitized hydrargillite obtained has a particle size smaller than 60 .mu.m. This material can be used to produce a rubber filler material for small mechanical loads.
Other fire-retardant plastic mixtures are used, for example in cables, in cable sheaths, and in cable insulation. The requirements to be met by such mixtures with respect to physical properties (mechanical and electrical properties) are extremely high. Only very fine-grained mineral flame-retardant agents (mean grain diameter smaller than 2 .mu.m) are able to meet such high requirements at present. In addition to the fineness of the grain size, the grain shape of the filler material also plays a decisive part. This is to ensure that in view of the high degree of filling, the viscosity level is kept moderately high during processing. High-quality commercial magnesium hydroxides which best meet the high requirements in force are relatively expensive. This is because they do not constitute mass products; and the production processes involved are complicated and expensive.
In the case of fire, the conductor sheaths, if consisting of the above material, have to meet stringent requirements. The fire-retardant plastic mixture, due to its high thermal stability, has to act as a mechanical spacer between the conductor wires. Even in the decomposed condition, it largely has to guarantee the insulating effect. This can only be achieved if the decomposition products occurring at the respective temperature of the fire form a relatively firm, stable protective layer.
From the literature reference (e.g. P. R. Hornsby, Macromol. Symp. 108 (1996) 203-219), it is known that one of the preconditions for being able to use a mineral filler material as a fire-retardant agent in a certain plastic system is based upon the relative decomposition temperatures of the polymer and the filler material. This becomes clear if one looks more closely at the various phases of a fire (see also Brandverhalten von Kunststoffen [Fire Behavior of Plastics]; author Juergen Troitzsch, publisher Carl Hanser Verlag, Munich, 1981, p. 11). A flame retarding agent has to play its part before the actual fire which is no longer controllable breaks out. This refers to the time between the start of the fire and the fully developed fire (characterized by the so-called flash-over). In such cases, flame retarding agents, because of their self-extinguishing properties, can either prevent a fire altogether or delay the outbreak of the fire sufficiently to allow plenty of time to minimize injury to people and damage to buildings. If the decomposition temperature of the filler material is clearly higher than the ignition temperature of the polymer, the contribution made by the filler to fire protection is not very high.